1. Field of the Invention
The present invention relates to an optical signal transmission apparatus and an optical signal transmission method, and more particularly to an optical signal transmission apparatus and an optical signal transmission method which use optical signals.
2. Description of the Related Art
With liquid crystal panels and plasma displays being provided with higher resolutions, it is desired that large-volume video signals from the host be transmitted whilst being kept in a digital format. For example, the DVI (Digital Visual Interface) standard, which has been defined by DDWG (Digital Display Working Group), an industry organization, is a standard for differential signaling called TMDS (Transition Minimized Differential Signaling) which transmits signals at a rate as high as 1.65 G bps per channel. As a medium for such high-speed signaling, metallic cable with a shield that is in wide spread use as a display cable is generally used, however, because of the high speed signaling, a problem that the length of the cable cannot be extended to over 10 m has arisen. An optical cable for realizing long distance transmission by converting this high-speed signal to light has been proposed, however, problems still to be solved are power consumption and safety in handling the laser light source.
As a technique which improves the handling safety of the laser light source, art which stops the laser light when the laser light leaks, as when disconnecting the optical connector, has been proposed (for example, Japanese Patent Application Laid-Open (JP-A) No. 5-244099/1993 and JP-A No. 2001-185783).
The art as disclosed in JP-A No. 5-244099/1993 proposes that an actuating signal for verifying the connection is sent in the direction opposite to the main-signal, which comprises video signals and the like. When the actuating signal for verifying the connection is not detected, on the main-signal sending apparatus side, sending of the main-signal is disabled.
The art as disclosed in JP-A No. 2001-185783 proposes that an electro-optical composite cable provided with electrically conductive terminals in the optical plug is used for connection, and when there is conductivity between the optical connector portions at both cable ends, the connection being verified, light is outputted.
As well as these, art as disclosed in national publication of the translated version No. 6-67391/1994 and others are proposed. With the art as disclosed in national publication of the translated version No. 6-67391/1994, it is proposed to check the link conditions of the respective sending and receiving modules of a full double optical fiber link, stopping the laser output when the reception signal is in a non-receiving status, and periodically illuminating the laser for prescribed periods of time with the use of a timer.
With the above-mentioned prior arts, however, the normal connection of the transmission medium is detected by only using a single method. This gives the problem that, if the optical connector is disconnected when the single method is malfunctioning for some reason, leakage of laser light can occur.
With the art as disclosed in JP-A No. 2001-185783, cost is increased because a specialized medium for connection verification is required.
Further, the art as disclosed in national publication of the translated version No. 6-67391/1994 uses a timer, thus the condition of the link is continuously inspected, even if there is no need to transmit a signal. This has an undesirable effect on the power consumption and the service life of the light emitting device.